AJT123

When does it "lock up" and can you tell? I have never figured this out.

TangoRed

In the most over simplistic terms, if you press the gas and the tach moves in direct proportion to the speed, it is locked up. If it rises to say, 2k rpm, and stays there as the car continues to gain speed it is not locked up (common scenario: moving from a stop). Different cars lock up at different times- the most efficient transmissions stay locked as much as possible.

AJT123

Gotcha!! That's sort of what I was guessing. It's "locked" when in a gear driving. I've had this explained to me before but I didn't get it.

You're good at Layman's terms!!!!

jwong77

Great conversation! just curious, how does the "lock up" technically work? Is it similar to how a manual transmission works where the clutch comes into contact with a pressure plate?

Stroock639

with the aisin transmissions it generally doesn't lock until you're cruising in the highest gear, but yes easiest way to tell is that the rpms and speed will be locked into a certain ratio, just like in a manual trans

the lockup is usually done through some kind of flex plate clutch

TangoRed

Thanks, glad you found that explanation useful.

There is a clutch involved actually! There is a solenoid (a type of electromagnet that converts energy into mechanical work) that when charged with electricity redirects fluid to put pressure against a clutch assembly that makes contact with the front of the torque converter. When this occurs, all of other the mechanisms that enable the torque converter to be unlocked and "slip" are taken out of the equation and you get the locked, direct drive state.

TangoRed

I'd say that's highly application specific. In today's market Aisin transmissions lock up as much as possible, with each new transmission touting a broader lock up range.

Stroock639

well yes but in the case of his cars we're dealing with 2 more old school aisin transmissions so that's what i meant... i should've been more clear

AJT123

Ok let's take this further. What the hell is a stall speed? I remember sport equipped E38 7 series had a "high stall speed torque converter". What does that mean?

mmarshall

Actually, the term you are looking for is called "Slippage". It means, as TangoRed alluded to in his first post, that the fluid-coupling in the transmission allows some "Slip" or excess engine-RPM, over the transmission's output-shaft. The engine will turn at a slightly higher RPM while cruising (and substantially higher RPM while accelerating) than if the system were locked, or with a straight mechanical hook-up like in a manual transmission. The torque converter locks up and prevents that slippage except at very low speeds or gear-ratios.....otherwise the engine would lug severely or stall. Some transmissions, in the quest for smoothness, used to allow an extreme amount of slippage, particularly on start-up from rest......Buick's old Dynaflow from the 40s and 50s, with no mechanical connections at all in the transmission (completely fluid-drive, with pitch-stators), was notorious for that, and for wasting fuel.

Chrysler was the first American manufacturer to introduce lock-up converters in modern times, in 1978, on its RWD Three-Speed Torqueflite-automatic-equipped vehicles. It did help with gas-mileage at a time when CAFE regulations were getting stricter, but, until the system was refined, was also the source of customer complaints from engine-lugging, and stalling if the system malfunctioned.

mmarshall

Stall speed is the engine-RPM that is needed before the power (torque) of the engine overcomes the drag in the torque-converter, gets the transmission's output-shaft spinning, and actually starts the vehicle moving. The higher the stall speed, the less efficient the transmission, and the more you will have to push the gas pedal to overcome the drag and get the vehicle moving. This is why, for automatic transmissions used in drag-racing, a low stall-speed is desired.....for quicker response when the light turns green and you want to start moving.

AJT123

Thank you Mike, for all of this info and the prior post too. Perhaps that sport package 7 series had a low stall speed instead of high and I was mistaken. In any case, it was marketed to make the 740i sport faster..

TangoRed

That's not totally correct info. Drag racers prefer high stall speed torque converters. It puts the engine in its powerband (ideally at the rpm where the engine creating maximum torque) while doing a brake-torque launch so the engine is creating maximum power right from the start.

In the case of the 740i, it allows the engine to be higher in its powerband and therefore create more power before the torque converter locks. Makes for a better launch.

​​​​​​​Ah ok that makes sense.

geko29

Other than the bit about being less efficient, this is completely wrong. Dedicated drag cars use high-stall torque converters ("mild" 9-10 second cars tend to use converters that stall between 3,500 and 4,500 rpm, the crazy-fast ones may use 6,000 rpm converters), because they want to be able to launch with the engine already in the powerband. The stall speed is where the the torque from the engine will force the transmission's output side to turn against a competing force like the service brakes (not simply the inertia of the vehicle's weight), or alternatively, the fastest the engine can possibly spin at full throttle if the output shaft is absolutely prevented from turning. If you use a nice, efficient 1,200 rpm converter on a drag car, you lug the crap out of the engine and get a terrible launch.

From folks who actually build these things for a living:

source

mmarshall

What you say is correct, but I think AJT123 was more interested in vehicles for regular street-use than in extreme conditions. Nevertheless, yes, I could have rephrased the way I used the term earlier.